In order to manufacture eyeglasses adapted to a wearer, the wearer usually choose a prebuilt frame, and the optician determines the features of the ophthalmic lenses in order for them to be positioned correctly in front of the eyes of the wearer, when he wears the chosen frame.
The prebuilt frame is usually tried on by the wearer before adding the ophthalmic lenses in order to check the adequacy of the dimensions of the frame with the shape of the face of the wearer.
The dimensions of the frame are indeed predetermined, and needs to fit the face of the wearer in order for the eyeglasses to rest in a comfortable and stable position on the face of the wearer, efficient for vision correction.
More precisely, one considers that the eyeglasses rest in an efficient and comfortable position when                the rims of the frame are positioned relative to the eyes of the wearer in a way that provides a large vision field to the wearer,        the eyeglasses frame rests on the nose of the wearer, without sliding or hurting him, in a stable fashion,        the branches of the eyeglasses frame rest on the ears of the wearer, without pressing on the temples, or pulling on the ears.        
Moreover, the dimensions of the frame must allow a correct placement of the ophthalmic lenses in front of the eye of the wearer, that is to say a correct centering of the ophthalmic lenses in front of the eye of the wearer.
Given all these criteria that should be fulfilled by the chosen eyeglasses frame, most frames come in different sizes. The wearer must then choose a design frame and then tries different sizes to determine which is best fitted to his face.
It is however difficult for the optician to carry many sizes for all the frames proposed in his office.
It is moreover very time-consuming to try on many different designs and sizes of frames in order to choose one that is adapted to the wearer.
Besides, because of the necessity to determine precisely if the frame fits the wearer and which size of frame would be best, it is difficult to sell eyeglasses online, since trying on the frame is not possible in this case.
Methods for virtually trying on eyeglasses frames are known, but do not provide sufficiently precise results.